Rolling strip



Patented June 15, 1937 UNITED STATES oUdl on not PATENT OFFICE ROLLING STRIP Application January 26, 1934, Serial No. 708,417

18 Claims.

My invention relates to the rolling of strip and, in particular, to the reduction of strip in the cold without annealing intermediate successive passes between the reducing rolls. Eyreduction in the cold, I mean rolling the strip in the absence of externally applied heat. Some heat is generated in the material itself during such rolling, but is not such as to alter its characterlsties materially.

It is recognized that it is advantageous in the rolling of strip to subject the strip entering the rolls to a restraining or back tension. Considerable difficulty has been experienced heretofore, however, in the application of the restraining tension to the entering strip. It has been attempted, for example, to apply the tension by a bridle engaging the strip, or by using fiat top and bottom guides on opposite sides of the strip, the bridle or guides being so adjusted as to yieldingly grip the strip, thus applying a frictional load and imparting a restraining tension. The restraining load applied to the advancing strip was dissipated to the strip itself in the form of heat generated by friction. Since it is impossible, as a practical matter, to secure perfectly uniform engagement between the bridle or guides and the strip across the width'of the latter, due to inevitable imperfections in the surfaces of the bridle or guides, and lack of uniformity in the strip itself, the frictional force on the strip and the heat gen erated thereby will vary from point to point across its width. The heating of the strip exaggerates the non-uniformity thereof, since the heating of the material tends to increase its thickness and the crux of the difiiculty is that the portions of the strip which were thicker to begin with are compressed by the guides or bridle, subjected to greater friction, are more highly heated, and are thus expanded to a greater extent than the originally thinner portions. The more highly heated portions of the strip caused corresponding hot zones in the rolls. This changed the pass contour of the rolls and so defeated the purpose of the bridle or guides, because the thicker portions of the strip were elongated more than the adjacent thinner portions. The disadvantage of this method and means for applying back tension become increasingly apparent with the continued reduction of the strip and increase in its speed through the rolls.

In cold rolling strip under tension, furthermore, a pronounced tendency to strip breakage has been observed. The hot rolled strip is always characterized by certain edge imperfections due to the hot rolling. It is found that when the strip is subjected to tension during cold rolling, the stress applied to the edge portions causes the edge defects to start breaks, which frequently result in a complete fracture of the strip. This is objectionable, not only because of the loss of material involved, but for the more important reason that the continuity of the processis broken and the production rate lowered.

In addition to the edge defects already mentloned, hot rolled strip may be characterized by a number of other imperfections. The strip may have camber at one or both ends in the same or opposite directions, that is to say, when laid out flat, the strip will not lie along a straight line but will assume a curve in one direction or the other, or a reverse curve if cambered in opposite directions. The hot strip, furthermore, may have loose or so-called wavy edges, indicating that the strip is thicker at the center than at the edges or, in other words, is crowned. The reverse may also be true, that is, the center of the strip may be full, while the edges are tight. This strip is buckled and assumes trough form when laid out flat, due to the over-rolled condition of the center. Where the contour of the section of the strip varies along its length, obviously, the effect of a constant tension on successive portions of a given length varies widely and the principal result is that the defects characterizing the hot strip become magnified in the cold rolled material.

I have invented a method of cold rolling strip which overcomes the aforementioned objections to such practice as it has been carried out heretofore. In accordance with my invention, I form a strip having fullness at the edges and cold roll the strip. Specifically, I impart to the hot rolled strip a crowned contour so that the edges are slightly over-rolled with respect to the central portion. This contour may be imparted to the strip by initially taking a comparatively heavy draft thereon, preferably by drawing the strip between small, backed-up reducing rolls, in the absence of any substantial restraining tension on the strip entering the rolls. The heavy draft causes slight bending of the rolls providing a crowned pass. The same result may be obtained by controlling the roll temperature to produce the desired contour. The strip is then again passed through the same or similar rolls adjusted to, take a lighter draft. The material is subjected to a substantial back tension during passes subsequent to the first. This tension is concentrated at the central portion of the strip, however, since the initial pass creates an over-rolled condition of the edges of the strip, which is apparent from the presence of rifiles therealong. This rifiled or over-rolled condition of the edges of the strip is maintained through the greater 5 part of the reducing cycle but is gradually reduced or corrected by the back tension. When the desired final gauge isapproached, the degree of back tension is so adjusted that the riflles disappear in the final passes and the resulting product is a flat strip of the desired gauge.

For a complete understanding of the invention, reference should be had to the accompanying drawings illustrating a preferred practice of the invention and showing diagrammatically the apparatus which may conveniently be used therein. In the drawings:

Figure l is a layout of a mill adapted for the performance of my method;

Figure 2 is a diagrammatic elevation of the mill rolls and their cooling apparatus;

Figure 3 is a plan view of the lower working and backing rolls with their cooling mechanism;

Figure 4 is a partial sectional view through the work rolls, the roll and strip contour being exaggerated considerably for the sake of clearness;

Figure 5 is a diagrammatic perspective view illustrating the condition of the edges of the strip during the cold rolling with all tension removed from the strip; and

Figure 6 is a graphical representation of the variation instrip gauge, the percentage reduction, the total back tension on the strip, and the unit back tension, during the several successive passes of the material between the rolls.

Referring now in detail to the drawings, I prefer to employ a four-high mill ll) of the roller bearing type having small work rolls I I and backing rolls l2 journaled in roller bearings, not shown. opposite sides of the mill, winding and unwinding reels I3 are located, having driving motors l4 connected thereto. Sheaves l5 between the mill and the reels guide the material in its passage back and forth therethrough. A motor 16 is disposed adjacent the mill for driving the rolls thereof. A

loader IT on the entering side of the mill facilitates the coiling of the material on one of the reels initially. A coiler l8 on the exit side of the mill removes the finished strip from the adjacent reel.

5 The reels 13 are provided with means for exerting a drag thereon when operating as unwinding reels. This drag may be applied by mechanical brakes but I prefer to utilize regenerative braking, operating the motors I4 as generators, as described and claimed in the U. S. Patent to McBain No. 1,881,056. The principal desideratum, in either case, is the dissipation of the energy absorbed from the advancing strip by the restraining means, at a point apart from the strip itself. As shown in Figure 1, the

motors [4 are arranged for regenerative braking,

having armature connections l9 to a common supply circuit 20. Field circuits 2|, including rheostats 22, are also connected to the circuit 20. It

will be understood that either motor l4, when operating as a generator, returns electrical energy to the circuit 26 to be consumed by the other motor I4, or any other load attached thereto. The

degree of back tension exerted on the entering strip is dependent, of course, upon the energy delivered by the motor operating as a generator,

to the load circuit, and this can be controlled by the adjustment of the rheostat 22.

As shown in Figure 2, the mill It] includes cooling boxes 23 in contact with the working and backing rolls adjacent their point of tangency. The cooling boxes are simply open troughs having connections 24 to a source of cooling fluid, and control valves 25 for regulating the fluid flow. As shown in Figure 3, cooling fluid supplied to the boxes through the connection 2-1 spreads toward the sides of the mill and, after flowing over the surfaces of the rolls, is discharged by leakage to a sump below the mill.

It is well known that it is possible to change the contour of the rolls slightly by controlling their temperature. I do this by varying the volume of cooling fluid directed thereto. As shown in Figure 2, there are four cooling boxes 23, each having its individual control valve 25. Because of natural radiation, the ends of the rolls are almost always cooler than the middle, but the temperature difference between the middle and ends of the rolls may be controlled, within limits, by the amount of cooling fluid delivered. A still greater range may be obtained by using a masking felt in the boxes having an opening which varies in width along the axis of the roll, so that a greater proportion of the roll surface is exposed to the fluid adjacent the middle of the rolls than at the ends.

In practicing the method of my invention, I preferably maintain the roll pass of the contour shown considerably exaggerated in Figure 4, substantially the same throughout the rolling operation. It will be apparent that, of the full width of the strip S shown between the rolls H, the central portion A, approximating two-thirds of the full width, is flat on both sides. The edge portions B, however, have a slightly tapering section, due to the contour of the roll pass obtained as explained above, by controlling the rate of flow of cooling fluid to the rolls. It will be apparent that the showing of Figure 4 is greatly exaggerated both as to the thickness of the strip S and as to the extent of taper in the edge portions thereof.

A roll pass contour of the character shown in Figure 4 imparts to the strip S, on rolling, a condition known as riffled edges. This condition is illustrated in Figure 5, from which it will be apparent that the central portion A of the strip passes through the rolls flat, while the edge portions 13 are over-rolled or wavy, presenting a full or riflled appearance when all tension is removed. I

The difference in the character of the surface of the central and edge portions may readily be observed during the rolling and is perhaps best described by the terms loose or wavy edges.

If back tension is applied to the strip S moving through the rolls in the direction of the arrow as shown in Figure 5, it will be apparent that by far the major portion of the tension will be sustained by the central portion A of the strip. Because of the loose or wavy condition of the edges of the strip, having considerable fullness therein, it will be necessary for the central portion of the strip to be elongated considerably before the edge portions will be subjected to any substantial tension. By maintaining the wavy condition of the edges, however, it is possible to relieve them of any considerable tension. Wavy edges, obviously, would not be a desirable characteristic in the finished product and I, therefore, contemplate adjusting the back tension so that as the desired final gauge of the strip is approached, the wavy condition of the edges will be removed by the combined flattening effect of the rolls and the back tension, and a substantially flat strip rolled as the final product. The back tension ap-' plied to the strip during rolling should ordinarily be sufflcient to make the strip entering the mill apparently flat.

A specific example of the method of my invention follows: I take hot rolled strip approximately .109" thick of any convenient width, for example, 36". The strip is received in coils from the hot mill and is placed on the loader IT for the first pass through the mill on to the reel l3 on the exit side of the mill which, during this pass, serves as a winding reel and applies forward tension to the material. During this first pass, the mill screwdowns are preferably set to take a heavy draft on the material, specifically, a reduction of about 25%, or to an initial gauge of .072" for the second pass.

I employ side guides during the first pass to insure that the strip follows the pass line or the horizontal axis of the mill in the rolling direction, and winds tightly on the coiling reel without any offsetting of successive turns. This permits subsequent passes to be made without side guides, which tension alone, even on both sides of the mill, will not make possible otherwise.

After the completion of the first pass, the strip is attached to the reel l3 on the entering side of the mill and pulled back through the mill for the second pass. On this pass, the unwinding reel (on the exit side of the mill) exerts a back tension on the entering strip by means of a mechanical brake or by regenerative braking, as already explained. In the case of the latter, the tension in the strip, for a constant speed, may be determined by measuring the output of the generator M or the input to the motor 16. This value may be converted into tension by the usual factors, the speed of the strip being measured in any convenient manner. The back tension should be suflicient to remove the riflied appearance of the edges so that an apparently flat strip is presented to the rolls, but should never exceed the elastic limit and may be rather light. Actually, of course, the riflies are still present but temporarily not visible due to the tension, except as variations in light and shade on the advancing strip. The emerging strip is also apparently flat due to the forward tension imparted by the coiling reel. On the second pass, a smaller reduction is preferably taken, for example, 15%. In a specific case, the total back tension was found to be in the neighborhood of 1200 lbs. on the second pass. This was equivalent to a unit back tension of about 800 lbs. per square inch of strip section. On the second pass, the gauge was reduced to .063".

The tension applied during the second and subsequent passes is confined to the central portion of the strip, as the riflled edges are maintained through the major portion of the reducing cycle or, at-least, until the final stages of the reduction are reached. The fullness of the riffled edges, as previously explained, precludes application of the back tension to the edges and it is found that strip breakage resulting from rolling material with edge defects under tension, is

practically entirely eliminated.

When rolling under tension the strip is directed accurately through the mill from one reel to the other, piling straight on both in successive passes. The side guides are unnecessary after the first pass and are therefore removed. The combined back and forward tension guides the stripso that the coils are straight and even.

The restraining tension stretches the shorter portions of the strip, e. g., the center, more than vuul VII the longer portions (such as the edges) producing a bodily flow of the metal which is the ideal sought in cold rolling. The roll contour tends to maintain the edges of the strip riflied as a precaution against breakage but the tension gradually removes the riflies and finally produces a fiat strip. The tension also permits the greater length of the loose or over-rolled portions to traverse the mill without folding over or triple-rolling. The accumulated excess of length would otherwise result in a fold which would not only mar the strip but is likely to start a break.

The curves in Figure 6, while not intended to represent the value of continuously varying functions, show the trends of the various factors in the successive rolling passes. These curves were obtained by plotting observed values such as have been mentioned. While a fairly uniform percentage of reduction is preferable throughout the rolling cycle, the actual reduction is much greater in the early passes than in the later ones, because the elastic limit rises as the rolling progresses. The initial heavy draft may be avoided and substantially the same percentage reduction employed in all passes. In such case, the roll contour will be relied on to establish the riflied edge condition.

The gauge of the strip decreases along a fairly regular curve to the final value. The total back tension also decreases, somewhat less regularly in the middle stages. The unit back tension, on the contrary, increases fairly regularly as the rolling proceeds, as required by the increasing elastic limit of the metal. It is the high unit back tension during the final passes which removes the edge riflies and results in the manufacture of a flat, cold rolled strip as the final product.

While some hot strip has a sectional contour approaching that which I prefer during the major portion of the rolling, in most cases, the hot strip will not have such a desirable characteristic. By taking a heavy reduction on the first pass, however, with the desired roll contour, I am able to impart to the hot strip the sectional shape mentioned, regardless of its previous condition. I consider the ideal shape of strip for rolling to be as shown in Figure 4, where the central portion of the strip, approximating two thirds of the full width, is flat, while the edge portions are tapered or over-rolled. Commercially flat hot strip is preferred because of the ease of loading it on the reel after the first pass.

It'is quite common that a coil of hot strip will have one characteristic at one end and another at the other, as previously indicated. Generallyspeaking, the ends of the coils of hot strip are characterized by greater defects than the central portions. I remove the end defects of the hot strip coils by varying the back tension in the early passes of the cold rolling. It is generally desirable to start the early passes with a fairly high tension, reducing the tension somewhat while rolling the middle portion of the coil, and again increasing the tension as the other end is approached. In this way, the end defects of the hot strip are gradually removed and a uniform condition established throughout the length of the coil. Generally speaking, a high back tension on the material entering the rolls tends to remove the edge rifile, For this reason, it is necessary to keepthe unit tension at a fairly low value during the early passes, since a full rolled condition of the center of the strip is the exact opposite of the optimum conditions as previously explained. Under a lower tension, the edges of the strip are over-rolled because of the contour of the rolls and possibly also partly because of the roll spring, with the result that the tension is confined or concentrated largely to the central portion of the strip.

It will be immediately apparent that the invention has numerous advantages over anything of the kind known to the prior art. In the first place, the energy absorbed from the advancing strip by the tension supplied thereto is dissipated apart from the strip and does not serveto exaggerate the variations in thickness across the width thereof by unequal heating and expansion of various portions.- The tearing or breaking of the strip during cold rolling as a result of back tension, is practically eliminated by this method. The percentage of reduction may be increased over that which has been practiced heretofore. Reduction at the rate of around 20% is easily possible, as compared to 10% representing previous practice. The loosening of the edge portions or bands of the strip relieves them of the greater part of the back tension which is therefore restricted to the central portion or band. The strip is never quite fiat during cold rolling before the final stage of the reduction. At that point, the unit back tension reaches a value sufficient to fiatten the emerging strip and remove all trace of the edge rifile therefrom. While the total tension is decreased as the rolling proprior art since it is known that strip breakage is caused by the forward tension used heretofore, and that the back tension is exerted on the strip leaving the mill as well as that entering the mill. Thus, back tension would obviously be thought to increase the danger of breakage but by the method of my invention, such danger is minimized.

The method of my invention has the added advantage that it eliminates the necessity of intermediate annealing between successive cold rolling passes. When sufficiently cold rolled, furthermore, the strip is characterized by the absence of stretcher strains and also has other desirable physical properties.

While the invention has been described specifically as applied to rolling in which the power is supplied principally by the forward tension in the strip, it will be recognized that it is also applicable to rolling in which power is applied to the rolls.

Although I have illustrated and described herein but one preferred method of practicing my invention and a preferred form of the apparatus therefor, with certain modifications, it will be obvious that many changes in the method and apparatus disclosed herein may be made without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, thereby exerting such pressure on the strip as to' produce rifiies at the edges thereof and maintaining them throughout the major portion of the reducing cycle, and, when the strip has been rolled nearly to the desired final gauge, applying such back tension as to remove the riflles and produce a substantially fiat strip of said gauge.

2. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, subjecting the strip to a back tension, dissipating the energy derived thereby apart from the strip, and over-rolling the edges of the strip slightly, thereby to concentrate the tension at the central portion of the strip.

3. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, reducing the gauge at least 15% on each pass, subjecting the strip to a back tension and. maintaining a rifiled condition of the edges of the strip to relieve them at least substantially of said tension until the later stages of the reducing cycle.

4. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, subjecting the strip during rolling to back tension, relieving the edges of the strip at least substantially from said tension, and decreasing the total back tension as the reduction of the strip progresses.

5. In a method of cold rolling strip, the steps including passing the strip between reducing rolls without intermediate annealing, subjecting the strip during rolling to back tension, and. confining the tension largely to the central portion of the strip to elongate the body thereof without subjecting the edges to substantial tensile stress.

6. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, subjecting the strip during rolling to back tension, increasing the tension While rolling the end portions of the strip and decreasing the tension while rolling the central portion.

7. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, effecting a reduction in gauge of at least 15% on each pass, subjecting the strip during rolling to back tension, controlling the roll contour to maintain the strip edges riflled during the early stages of the reduction, and controlling the tensiopin the later stages to prod'licea fla rip o the -desired final gauge.

8. In a method of making flat, thin strip from stock having a center buckle, the steps including passing the stock between reducing rolls, subjecting the stock to a heavy draft, controlling the contour of the rolls to remove the bimeana cause riffling of the edges of the stock sufficient to relieve the edges of tension applied thereto in subsequent passes but insufficient to cause pinching of the edges, maintaining the edge riffling as the reduction proceeds, and applying back tension to the strip sutlicient to remove the riffling before the desired final gauge is reached.

9. In a method of making thin, fiat strip from stock having a non-uniform thickness across the width thereof, causing tight and loose bands therein, the steps including passing the stock between reducing rolls, controlling the roll pressure to loosen the edge bands of the stock, and subjecting the stock to back tension during rolling sufficiently to elongate permanently the central portion thereof.

10. In a method of rolling strip material, the steps including passing it between reducing rolls, controlling the roll pressure to riflle the edges of the strip and subjecting the strip to back tension to elongate permanently the center portion of the strip and remove the edge riffles before the desired final gauge is reached.

11. In a method of rolling strip, the steps including passing it repeatedly between reducing rolls, initially maintaining a fullness in the edges of the strip, and subjecting the strip to back tension sufficient to elongate permanently the center of the strip and remove such fullness.

12. In a method of rolling strip, the steps including passing it between reducing rolls, initially maintaining a fullness in the edges of the strip, subjecting the strip to back tension sufficient to elongate the center of the strip permanently, and increasing the unit back-tensile stress as the reduction progresses, finally to remove said fullness and produce a flat, finished strip.

20 13. In a method of rolling, the steps including forming a strip having fullness at the edges, and cold rolling the strip under tension suificient permanently to elongate the central portion thereof.

25 14. In a method of making strip, the steps including passing stock between reducing rolls, subjecting the stock entering the rolls to a back tension to assist in the elongation thereof, controlling the pressure exerted by the rolls to pro- 30 duce a fullness in the edges of the stock at least initially, and adjusting the back tension to elongate permanently the central portion of the stock and remove substantially all such fullness before the stock is reduced to the desired final gauge.

15. In a method of making fiat sheet metal in strip form, the steps including rolling a blank having a crowned or convex contour, passing the blank repeatedly between reducing rolls, subuu'wl Ulll HUU jecting the blank to back tension during rolling, and so maintaining the contour of the blankduring rolling as to rifiie the edges thereof and prevent said edges from sustaining any substantial portion of said back tension, at least until the blank has been reduced to a thickness in the neighborhood of the desired final gauge.

16. In a method of cold rolling strip, the steps including passing the strip between reducing rolls without intermediate annealing, producing fullness at the edges of the strip, maintaining such fullness during the greater part of the rolling, applying back tension to the strip during rolling, and increasing the tension toward the end of the rolling to such an amount as to remove said fullness from the edges of the strip.

17. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, adjusting the pressure exerted by the rolls to produce fullness at the edges of the strip, maintaining such fullness during a portion of the rolling, and removing the fullness by applying back tension to the strip entering the rolls.

18. In a method of cold rolling strip, the steps including passing the strip between reducing rolls, subjecting the strip on the initial pass to a heavy draft in the absence of substantial back tension, to create an over-rolled condition at the edges of the strip, and on subsequent passes, subjecting the strip to lighter drafts, tensioning the advancing strip at the middle thereof without substantially stressing the edges thereof, dis sipating the energy derived by tensioning apart from the strip, and, when the desired final gauge has been nearly reached, varying the tension on the strip to produce a substantially fiat strip of said final gauge.

HOWARD STANLY LAMB. 

